Field of the Invention
The present invention relates to a high-frequency power supply device for generating plasma by supplying high-frequency power to an induction coil included in a resonant circuit from a DC power supply via a switching circuit.
Description of the Related Art
For example, an analysis device such as an inductively coupled plasma (ICP) emission spectrometer has a configuration of generating plasma at a plasma torch by supplying high-frequency power to an induction coil included in a resonant circuit (for example, see JP-A-10-214698, JP-W-2009-537829, JP-A-6-20793, and WO 2012/039035 A). A high-frequency electromagnetic field is generated by the high-frequency power supplied to the induction coil, and an induced current flows by charged particles in plasma being accelerated, and plasma is thereby heated.
In the case of such a configuration, the impedance (resistance component and reactance component) of the induction coil is changed in accordance with generation of plasma. That is, the effective inductance of the induction coil is reduced due to reduction, caused by the induced current, in the magnetic field formed by the induction coil. Also, due to loss of energy caused by heating of plasma, a resistance component is caused in the induction coil. Furthermore, the impedance of the induction coil also changes due to the state of plasma changing according to the state of a plasma-generating gas or an analysis sample, the plasma input power or the like.
At the time of inputting power to plasma, the resonant circuit composed of the induction coil and a capacitor is driven at a specific oscillation frequency. The output impedance of a normal high-frequency power supply is set to 50Ω, and thus an impedance conversion circuit is arranged between the high-frequency power supply and the induction coil, and control is performed in such a way that the impedance seen from the high-frequency power supply side is 50Ω at all times. In this case, to eliminate the reflected power from the impedance conversion circuit, a method is generally adopted of driving a vacuum variable capacitor in the impedance conversion circuit by a motor or the like and adjusting the capacitance, for example.
In the case of such a configuration, the plasma input power is equal to the output power of the high-frequency power supply, and thus by calibrating the output power of the high-frequency power supply in advance by using a power meter with 50Ω, for example, the plasma input power may be accurately controlled. However, with such a configuration, to maintain the optimum state at all times by controlling the impedance conversion circuit, a complex control mechanism and expensive parts have to be used. Accordingly, these days, a method of changing the frequency according to a change in a load impedance (a so-called free running method) without using expensive parts such as a vacuum variable capacitor is being widely used.
JP-A-10-214698, JP-W-2009-537829, JP-A-6-20793, and WO 2012/039035 A disclose techniques related to a self-oscillating method as the simplest circuit configuration of the free-running type. These techniques do not limit the output impedance of the high-frequency power supply to 50Ω, and adopt a method of directly driving the resonant circuit composed of the induction coil and the capacitor. By adopting a self-oscillating method where the frequency automatically changes according to a change in the load impedance, as described above, a frequency control circuit, an impedance conversion circuit and the like may be omitted, and a simpler high-frequency power supply device may be provided.